Compound transmissions of the range or combined range/splitter type are well known in the prior art. Such transmissions typically comprise a multiple speed main transmission section connected in series with a range type auxiliary section wherein the range step is greater than the total ratio coverage of the main transmission section.
In such transmissions, the main section is typically shifted by means of a shift housing assembly controlled by a manually operated shift lever or the like. In contrast, the auxiliary range section is shifted by means of a button or switch. In the prior art, a switch operated by the main section shift mechanism controls a remote slave valve/actuator mechanism. The valve/actuator mechanism controls operation of a range selection actuator. The range selection cylinder includes a fluid-actuated piston that divides an actuator cylinder into a low-range and a high-range chamber. The piston is made to move in response to selective pressurization of one chamber while simultaneously exhausting a second chamber. Since the range section often utilizes synchronized jaw clutches, a range shift is preferably initiated and completed while the main transmission section is in neutral, to provide acceptable shift quality and to prevent undue wear and/or damage to the synchronized jaw clutches.
Prior art compound range type transmissions usually include a control system, which is typically a pneumatic control system having interlock devices, that allows a range shift to be preselected using a selector button or switch at a master control valve. To prevent damage to the jaw clutches, the control system prevents shift initiation until the main transmission section is shifted to, or at least towards, the neutral condition. One method for preventing shift initiation utilizes mechanical type interlock devices on the range section actuator mechanical linkage that physically prevent movement of the range section shift fork until the main section is shifted into neutral. A second, more common method of preventing shift initiation utilizes logic-based interlock devices of the type wherein the valve supplying pressurized fluid to the range section cylinder is either disabled or not provided with pressurized fluid until a shift to main section neutral is sensed, or is only activated and provided with pressurized fluid while the main section is shifted to and remains in neutral. Examples of such transmissions and the control systems therefor may be seen by reference to U.S. Pat. Nos. 2,654,268; 3,138,965, 4,060,005 and 4,974,474.
Prior art systems are particularly concerned with protecting the range synchronizer during range upshifts, or upshifts to the high range. It has been discovered that for certain systems, successful completion of a high range shift is more likely to be achieved in a timely fashion relative to the shifting of the main section. In such transmissions, it is highly desirable to provide a mechanism for protecting the low range synchronizer.